Investment casting with improved melt feeding

ABSTRACT

Method and apparatus for casting of molten metallic material wherein the metallic material is introduced into a mold and a pressure cap then is clamped on the mold by a cam/cam follower mechanism to form a seal such that pressurizing gas can be introduced into the mold on the molten metallic material therein. The pressure cap includes an inlet for pressurizing gas, which is introduced on the molten metallic material in the mold after the pressure cap is sealed thereon. Application of gas pressure on the molten metallic material in the mold improves filling of mold with the molten material and reduces or eliminates non-fill voids in the solidified casting.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to casting of metals and alloys in a mold in a manner to apply localized gas pressure on the melt in the mold after it is cast into the mold to improve filling of mold and reduce or prevent non-fill voids in the solidified casting.

BACKGROUND OF THE INVENTION

[0002] In the manufacture of turbine blades and vanes for modern, high thrust gas turbine engines, there has been a continuing demand by gas turbine manufactures for internally cooled blades and vanes having complex, internal cooling passages including such features as pedestals, turbulators, and turning vanes in the passages in a manner to provide desired cooling of the blade or vane. These small cast internal surface features typically are formed by including a complex ceramic core in the mold cavity in which the melt is cast. The presence of the complex core having small dimensioned surface features to form pedestals, turbulators, and turning vanes or other internal surface features renders filling of the mold cavity about the core with melt more difficult and more prone to inconsistency. wettable ceramics and increased metallostatic head on the mold and higher temperatures have been used in an attempt to improve mold filling and reduce localized voids in such situations, but these are costly and may be restricted by physical size of the casting apparatus.

[0003] U.S. Pat. No. 5,592,984 describes a method of investment casting gas turbine engine blades and vanes and other components wherein a ceramic investment mold is disposed in a casting furnace in a vacuum casting chamber and filled with the melt. The vacuum casting vacuum chamber is gas pressurized rapidly enough after filling of the mold with melt and prior to withdrawal of the mold from the casting furnace to reduce localized void regions present in the melt as a result of surface tension effects between the melt and mold components such as ceramic mold and/or core.

[0004] U.S. Pat. No. 6,019,158 and 6,070,644 of common assignee herewith describe use of pressure cap that is pivoted and sealed on the pour cup of an investment shell mold to apply gas pressure to the melt residing in the pour cup to improve filling of fine mold/core details with the melt.

[0005] It is an object of the present invention to provide a method and apparatus for casting a molten metallic material where gas pressure is provided on the melt in the mold in a manner to improve filling of the mold with the molten metallic material.

SUMMARY OF THE INVENTION

[0006] The present invention provides method as well as apparatus for casting of molten metallic material wherein the molten metallic material is introduced into a mold and a pressure cap then is clamped on the mold using a cam/cam follower mechanism such that pressurizing gas can be introduced into the mold on the molten metallic material therein. The pressure cap includes a seal-carrying member disposed on the mold and a rotatable cam member disposed on the seal-carrying member. The seal-carrying member includes a compressible seal that seals against a surface of the mold, such as a lip of a mold pour cup. One or more cam surfaces are provided on the cam member and actuate one or more cam followers that engage(s) a respective cam surface and also the mold in a manner that the seal is compressed on the mold surface between the seal-carrying member and the mold when the cam member is moved. The pressure cap includes a gas inlet for pressurizing gas, which is introduced on the molten metallic material in the mold after the pressure cap is clamped thereon. Application of gas pressure on the molten metallic material in the mold improves filling of mold with the molten metallic material and reduces or eliminates non-fill cavities or voids in the solidified casting.

DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is an elevational view of a pressure cap of the invention on a pour cup of an investment shell mold for practicing a method of the invention.

[0008]FIG. 2 is a plan view of the pressure cap.

[0009]FIG. 3A is an enlarged, partial elevational view, partially broken away, of the cam/cam follower mechanism of the pressure cap of FIG. 1. FIG. 3B is an enlarged, partial side elevational view of the mechanism of FIG. 3A taken 90 degrees to FIG. 3A. FIG. 3C is a partial plan view of the mechanism.

[0010]FIG. 4 is a partial plan view of the seal plate member showing the upstanding brackets.

[0011]FIG. 5 is a partial perspective view of the cam/cam follower mechanism.

DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention provides method and apparatus for casting of metals and alloys and is especially useful in investment casting of nickel, cobalt and iron base superalloys with equiaxed, single crystal, or columnar grain microstructures as well as titanium and its alloys and other commonly used metal and alloys. The present invention can be practiced to make equiaxed, single crystal, or columnar grain castings which may be cored or not to produce complex internal passages in the castings.

[0013] Referring to FIGS. 1-5, apparatus in accordance with an illustrative embodiment of the invention for casting a molten metallic material in an investment shell mold assembly 10 is illustrated. The shell mold assembly 10 comprises a mold cluster having a plurality of shell molds 12 (two shown) each with a mold cavity 12 a, which is filled with melt that is solidified to form a casting in each mold cavity. The mold cavities 12 a each may have an optional ceramic core (not shown) positioned therein to form internal passages and other features in the casting.

[0014] The shell molds 12 are disposed about a common frusto-conical pour cup 12 b and adapted to receive the molten metallic melt via the open bottom 120 of the pour cup and respective sprues or runners 12 c that are communicated in melt flow relation to the pour cup and each mold cavity 12 a.

[0015] The mold 10 typically comprises a ceramic investment shell mold cluster having the features described above and formed by the well known lost wax process wherein a wax or other fugitive pattern of the mold assembly is dipped repeatedly in ceramic slurry, drained, stuccoed with coarse ceramic stucco and dried to build up the desired shell mold thickness on the pattern. The pattern then is removed from the invested shell mold, and the shell mold is fired at elevated temperature to develop adequate mold strength for casting. Investment shell molds formed in this manner exhibit porosity and substantial permeability to gas as a result. The mold assembly 10 is cast in a vacuum casting chamber (not shown) and then removed from the vacuum casting chamber for cooling to solidify the molten metallic material in the shell molds 12. In particular, a conventional tiltable induction melting crucible (not shown) is disposed in a vacuum casting chamber for heating and melting the charge of metal or alloy to form the melt to be cast. The melt typically is heated to a superheat temperature selected in dependence on the metal or alloy being cast. The melt is poured from the crucible into the pour cup 12 b of the mold 10 and flows through runners 12 c into the mold cavities 12 a of the molds 12. The mold assembly 10 is filled to provide a molten metal or alloy level L in the pour cup 12 b to provide a metallostatic head in conventional manner.

[0016] After the mold assembly 10 is cast in the vacuum casting chamber, the chamber vacuum is broken and the melt-filled mold 10 is removed from the casting chamber into the ambient air atmosphere. The melt-filled mold 10 is supported and held stationary in the ambient air atmosphere on a U-shaped collar CL of a stationary fixture.

[0017] Pursuant to an embodiment of the invention, a pressure cap 30 is clamped on the melt-filled mold assembly 10 immediately upon its removal from the vacuum casting chamber, and a pressurizing gas is introduced into the mold pour cup 12 b in the space S above the upper level or surface L of the molten metallic material M therein until the molten material in mold cavities 12 a at least partially solidifies. The mold pour cup 12 b and runners 12 c are wrapped with thermal insulation (not shown) upon removal of the mold assembly 10 from the vacuum casting chamber to retard cooling of the molten material in these regions of the mold 10.

[0018] For example, the pressure cap 30 is clamped on the melt-filled mold assembly 10 within about 2 to about 5 seconds after removal from the vacuum casting chamber, and an inert, non-reactive or other pressurizing gas is introduced into the space S on top of the upper level or surface L of the molten metallic material M in the pour cup 12 b. The gas pressure applied on the molten material in the pour cup 12 b typically is up to 15-20 psi superambient depending on shell strength with a gas pressure of 5 to 10 psi superambient being sufficient for certain applications. An inert gas, such as argon, is a suitable pressurizing gas for casting conventional nickel base superalloys. The gas pressure is applied on the molten material in the pour cup 12 b for a time of several minutes, such as for example, for a molten nickel base superalloy, a time of 2-3 minutes until the superalloy solidifies completely in the mold cavities 12 a. The gas pressure can be terminated after this time (e.g. 2-3 minutes), and the pressure cap 30 removed from the mold assembly 10. The mold assembly 10 then can be allowed to cool to ambient temperature in ambient air over a time period which may take 10 hours or more. The mold assembly 10 then is subjected to a conventional knock-out operation to remove the castings from the shell molds 12.

[0019] The pressure cap 30 includes a non-rotatable seal-carrying plate member 38 disposed on the mold 10 and having an annular, compressible refractory gasket seal 40 on its underside. The refractory gasket seal 40 can comprise a commercially available ceramic fiber or needled material used for high temperature gaskets. The gasket seal 40 is disposed on the underside of the plate member 38 by mechanical clips (not shown) located on the underside of plate member 38, adhesive, or by any other seal fastening technique. The gasket seal 40 is adapted to engage and seal against the annular lip surface 121 of the pour cup 12 b of the melt-filled mold assembly 10. The seal-carrying plate member 38 has first and second support frames 34 each welded at welds WW (one shown) or otherwise attached on opposite diametral sides thereof. Each support frame 34 comprises a pair of spaced apart upstanding brackets 34 a with a pair of guide rods 34 b being disposed between the brackets 34 a as shown in FIGS. 3A, 3B, and 4. The guide rods 34 b are spaced one above the other in a common vertical plane in the space between the brackets 34 a.

[0020] The pressure cap 30 also includes a cam mechanism 32 to clamp the pressure cap on the melt-filled mold 10 with gasket seal 40 engaging the annular lip surface 121. The cam mechanism 32 includes a rotatable cam plate member 36 that is disposed on top of seal plate member 38 for rotation relative thereto. The cam plate member 36 and seal-carrying plate member 38 are shown as having a circular profile overlying the open upper annular lip 121 of the pour cup 12 b for purposes of illustration only as other shapes are possible. The cam plate member 36 carries a plurality (e.g. two shown) of cam surfaces 42 on the upper side of the cam plate member 36. The cam surfaces 42 are engaged by a respective cam follower mechanism 41 disposed on a respective one of the support frames 34. The cam surfaces 42 are machined on cam blocks 43 attached to the cam plate member 36 by welding or mechanical fastening such as screwing. Each cam surface 42 includes a region 42 a that inclines in the direction of arrow A at an angle of 30 degrees for purposes of illustration only and a flat horizontal region 42 b terminating at an upstanding stop projection 42 c.

[0021] Each cam follower mechanism 41 comprises a cam follower surface, such as follower wheel 44 comprising an outer bearing race 44 a and inner bearing race 44 b fixed on screw 52, FIGS. 3A, with antifriction ball bearings 44 e between the races 44 a, 44 b so that the cam follower wheel outer race 44 a rotates relative to screw 52. Screw 52 includes a threaded shaft portion 52 a threadably received in upstanding follower member 45 to fasten the follower wheel 44 on follower member 45, which can comprise an elongated, rectangular bar-shaped member. The head 52 b of screw is shown only in FIGS. 3A, 3C for convenience. The follower member 45 is received in the space between the brackets 34 a as shown best in FIG. 5 for up and down movement therein. The follower member 45 includes an upstanding slot 45 s that receives the guide rods 34 b disposed between the brackets 34 a to guide movement of the follower member 45 upwardly and downwardly between the brackets 34 a as the cam plate member 36 is rotated. Each follower member 45 includes a lower end having a mold-gripping surface 46, such as a cylindrical rod 46 a, welded or otherwise attached to the lower end to engage opposite sides of the outwardly diverging frusto-conical wall 12 w of the pour cup 12 b as shown in FIGS. 1 and 3B. The cam plate member 36 includes a radially extending manually-operable handle 54 by which the cam plate member is rotated by an operator (worker) relative to the non-rotatable seal plate member 38 in the direction of arrow CW to clamp the pressure cap 30 on the melt-filled mold 10 with the gasket seal 40 pressed on the lip surface 121 of the pour cup 12 b. Rotation of the seal-carrying plate member 38 is prevented by the operator's gripping and holding handle 30 welded or otherwise attached to the side of the plate 38 while the handle 54 is rotated relative thereto. The stops 42 c on cam surfaces 42 limit the rotational motion of the cam plate member as a result of each cam follower wheel 44 abutting stop projection 42 c.

[0022] The pressure cap 30 is clamped on the melt-filled mold assembly 10 by placing frames 34 on opposite exterior sides of the frustoconical pour cup 12 b with the cam plate member 36 and seal plate member 38 overlying the pour cup lip 121 and with mold-gripping surfaces 46 engaged with the pour cup wall 12 w as shown. The mold 10 and pressure cap 30 are properly positioned relative to one another by the mold pour cup 12 b engaging a depending stop 61 welded or otherwise attached on the underside of the handle 30, FIG. 1. The cam plate member 36 then is rotated by an operator manually moving handle 54 in the clockwise direction relative to handle 30 to move cam surfaces 42 relative to cam follower wheels 44. The follower members 45 thereby are moved upwardly by cam surfaces 42 while the mold-gripping surfaces 46 engage or grip the mold such that the seal-carrying member 38 is urged toward the mold 10 when the cam member 36 is so rotated, thereby compressing the gasket seal 40 on the pour cup lip 121 to provide a seal therebetween. The compressed gasket seal 40 is somewhat gas permeable such that perfect gas tight seal is not provided; yet the sealing action provided by the compressed gasket seal 40 is effective enough to permit the supply of pressurizing gas to overcome any gas loss as a result of permeability of the gasket seal to thereby provide the desired gas pressure on the upper level or surface L of the molten metallic material in the pour cup 12 b. A pressurizing gas is introduced into the space S in pour cup 12 b above the molten metallic material M therein through a threaded gas inlet fitting 62. The inlet fitting 62 extends with clearance through an aperture 36 a in the cam plate member 36 and is threaded in a threaded bore provided in the plate member 38 to communicate with the space S in the pour cup 12 b above the level L of molten metallic material M therein. The fitting 62 is connected by conduit or line 64 to a source GS of pressurizing gas at superambient pressure. The source of pressurizing gas can comprise a conventional argon supply cylinder having a pressure regulator 66 and pressure relief valve 68 to provide argon gas pressure on the molten material in the pour cup 12 b for 2-3 minutes or other appropriate time until the molten material solidifies in the molds 12. After the selected time period, the gas pressure to the gas inlet 62 is terminated by shutting off the argon supply. The handle 54 then is rotated relative to handle 30 by an operator in the counterclockwise direction to move cam surfaces 42 in the direction to release the camming action of the cam plate member 36 on seal-carrying plate member 38. The frames 34, cam plate member 36, and plate member 38 then are released from the pour cup 12 b to allow removal of the pressure cap 30 from the pour cup 12 b. The mold assembly 10 then is subjected to continued cooling in ambient air to ambient temperature. The solidified castings in molds 12 then can be removed by a conventional knock-out operation to remove molds 12 therefrom.

[0023] The present invention facilitates the filling of details in the mold cavity that are defined by internal mold surface features and/or core surface features that are otherwise difficult to fill with the molten metallic material M. The present invention is advantageous to reduce or eliminate non-fill voids in the castings solidified in the molds 12. It is to be understood that the invention has been described with respect to certain embodiments thereof for purposes of illustration and not limitation. The present invention envisions that modifications, changes, and the like can be made therein without departing from the spirit and scope of the invention as set forth in the following claims. 

I claim:
 1. Apparatus for casting a molten metallic material, comprising: a mold having a mold cavity to receive the molten metallic material, and a pressure cap having a seal-carrying member disposed on said mold and having a cam member disposed on the seal-carrying member, said seal-carrying member including a seal for sealing on a surface of said mold, said cam member having a cam surface, said pressure cap including a cam follower that engages said cam surface and said mold in a manner that said seal is compressed against said surface between said seal-carrying member and said mold when said cam member is moved, said pressure cap including a gas inlet for introducing pressurizing gas into the mold on a surface of the molten metallic material therein.
 2. The apparatus of claim 1 wherein said cam follower is disposed for movement on said seal-carrying member.
 3. The apparatus of claim 2 wherein said cam follower includes a follower surface that engages the cam surface and a mold-gripping surface that grips the mold in a manner to urge said seal-carrying member toward said mold when the cam member is rotated.
 4. The apparatus of claim 3 wherein the mold-gripping surface engages a frusto-conical pour cup of the mold.
 5. The apparatus of claim 3 wherein said cam member includes a pair of said cam surfaces that are engaged by a respective said cam follower.
 6. The apparatus of claim 3 wherein said cam follower includes an upstanding follower member having the follower surface and the mold-gripping surface thereon, said cam follower member being disposed for up and down movement between a pair of brackets connected to said seal-carrying member.
 7. The apparatus of claim 1 where the seal resides on a lip of a mold pour cup so as to seal thereon when the cam member is moved.
 8. The apparatus of claim 1 wherein said cam member is circular cam plate and said seal-carrying member is a circular plate having said seal on a side facing the mold.
 9. The apparatus of claim 8 wherein the cam member includes a handle by which it is rotated relative to the seal-carrying member.
 10. The apparatus of claim 1 including a source of inert gas communicated to said gas inlet.
 11. The apparatus of claim 1 wherein said gas inlet extends with clearance through an aperture in the cam member and is connected to the seal-carrying member to communicate to a mold interior.
 12. Method of casting, comprising introducing molten metallic material into a mold, placing a pressure cap on the mold after the melt is introduced therein with a seal of said pressure cap disposed on a mold surface, camming the pressure cap toward the mold to compress the seal against the mold surface, and introducing gas through an inlet in the pressure cap on the melt residing in the mold to provide gas pressure on said melt.
 13. The method of claim 12 wherein the pressure cap is cammed toward the mold by movement of a cam follower that engages a cam surface on the pressure cap and that also engages the mold.
 14. The method of claim 13 wherein the cam surface is moved relative to the cam follower while the cam follower engages the mold.
 15. The method of claim 13 wherein the cam follower engages a frusto-conical pour cup of the mold.
 16. The method of claim 12 wherein an inert gas is introduced through the inlet into the mold.
 17. The method of claim 12 wherein the molten metallic material is introduced into the mold in a vacuum casting chamber, the mold is removed from the vacuum casting chamber, and the pressure cap is placed on the mold after the mold is removed from the vacuum casting chamber. 